Plate Tectonics part II

1
Plate Tectonics (part II)

• Sea-Floor Spreading

2
Sea Floor Spreading

• 1930-1950s the theory of Continental Drift was
  set aside
• Alfred Wegener was not able to convincingly prove
  his theory because of the lack of a mechanism
  that can make continents move
• Renewed interest in Continental Drift after the
  discovery of rock magnetism ( a field of study in
  physics)

3
Sea-Floor Spreading

• Paleomagnetism Magnetism preserved in rocks

4
Sea Floor Spreading

• Earths magnetic Field generated by the liquid
  outer core
• Magnetic Field -the portion of space near a
  magnetic body where magnetic forces due to the
  body can be detected

5
Sea Floor Spreading

• Earths magnetic Field generated by the liquid
  outer core
• Magnetic Field -the portion of space near a
  magnetic body where magnetic forces due to the
  body can be detected

6
Sea Floor Spreading

• Curie point (580 deg centigrade) minerals lose
  their magnetism
• Below 580 degrees centigrade get magnetized

7
Sea Floor Spreading

• Magnetic minerals align themselves parallel to
  the magnetic lines of force

8
Sea Floor Spreading

• Magnetic minerals get preserved in rocks and
  record the prevailing magnetic field at the time
  the rock was formed.

9
Sea Floor Spreading

• Declination
• The angle of alignment of magnetic minerals to
  the Earths surface
• What if the declination changes?

10

• Polar wandering
• A study conducted in Europe by S.K. Runcorn
• Magnetic alignment of lavas with different ages
  was found to vary widely

11
Sea Floor Spreading

• New data rekindled interest but did not cause a
  major swing in opinion
• The techniques used in extracting paleomagnetic
  data was new and untested
• Rock magnetism tends to weaken with time

12
Sea Floor Spreading

• During the 1950s and 1960s advances in
  technology permitted extensive mapping of the
  ocean floor
• Discovery of the a global oceanic ridge system
• Rocks are older farther from the oceanic ridge
• High heat flow characterized the oceanic ridge
  systems
• Earthquakes occurred in trenches

13
(No Transcript)
14
Sea Floor Spreading

• Early 1960s Harry Hess put together these
  observations into a hypothesis
• He presented his paper as an essay in geopoetry
• Unlike its predecessor, the continental drift
  hypothesis, sea floor spreading centered on the
  processes that transpired on the ocean floor.

15
Sea Floor Spreading

• Harry Hess proposed that the ocean ridges were
  areas where new oceanic crust formed.
• Older rocks that formed earlier moved farther
  away from the oceanic ridges as the sea floor
  spread

16
(No Transcript)
17
Sea Floor Spreading

• Geomagnetic reversals
• Physicists accepted the fact there were periodic
  magnetic reversals that occurred during Earths
  history
• A rock solidifying during one of the periods of
  reverse polarity will be magnetized with the
  opposite polarity of rocks being formed today.
• Reverse polarity and Normal polarity

18
Sea Floor Spreading

• Geomagnetic reversals (evidence)
• Lavas
• Sedimentary rocks of the ocean floor

19
Sea Floor Spreading

• Geomagnetic reversals - evidence from lavas and
  sedimentary rocks of the ocean floor

20
Sea Floor Spreading

• Interpretation of magnetic stripes came from Fred
  Wine and Drummond Mathews
• As new rock is formed at the oceanic ridge, its
  magnetic components take the polarity of the
  existing magnetic field.

21
Sea Floor Spreading

• We should expect a pattern of stripes to both
  sides of the spreading floor (left and right
  sides of the oceanic ridge).
• Magnetic stripes correspond to the normal and
  reverse polarities as recorded in oceanic crustal
  rocks.

22
Sea Floor Spreading

• Paleomagnetism was the most convincing evidence
  in support of continental drift
• By 1968 scientists began reversing their stand on
  continental drift like a magnetic reversal